I stand to be corrected, since numbers are just about the only words I know in that language, but I believe that Japanese has a very simple number system. Translated into English, it goes as follows:

1 to 10 - unique(*see below) words for each
11 - ten-one
12 - ten-two
13 - ten-three
...
20 - two-tens
21 - two-tens-one
22 - two-tens-two
...
31 - three-tens-one
...
99 - nine-tens-nine
100 - dunno! My grandfather only taught me to 99 (the number I mean, not his age!)

*Mind you, I understand that they then proceed to throw away all that simplicity by having different forms of each word (e.g. several different variations of the word for "four") depending on what is being enumerated.